1. Field of the Invention
This invention relates to a hydrogen absorbing alloy powder suitable for use in an environment where the alloy powder is subject to corrosion, such as alkaline secondary batteries.
2. Description of the Related Art
Generally, hydrogen absorbing alloys are not used in the form of an as-cast ingot, but in the form of a powder.
In the prior art, therefore, various methods for reducing a cast alloy ingot to an alloy powder having a desired average particle diameter have been employed. For example, the alloy ingot is reduced to coarse particles by primary pulverization with a jaw crusher or the like, and then to fine particles by secondary pulverization with a Brown mill or the like, followed by classification with a sieve. Alternatively, the alloy is formed into coarse primary particles, for example, by gas atomization, and then subjected to secondary pulverization with a pin mill or the like, following by classification with a sieve.
In all of these methods, the prepared alloy is ground by the action of mechanical impact, so that finely divided particles are produced during pulverizing. When the resulting pulverized product is classified to obtain a powder having a desired average particle diameter, this powder contains very large amounts of finer and coarser particles as compared with those of average diameter, as shown in FIGS. 4 and 7.
When an alloy powder containing large amounts of particles having diameters different from the desired one is used in an environment where the alloy powder is subject to corrosion, for example, in an alkaline secondary battery where the alloy powder is used as an electrode active material in a highly concentrated aqueous alkaline solution, its corrodibility by the aqueous alkaline solution used as the electrolyte (i.e., the ease with which alloy components dissolve from the alloy particles into the electrolyte), and its ease of charging and discharging (i.e., the ease with which hydrogen is absorbed into the alloy and released from the alloy) vary greatly according to the size of alloy particles.
A s a result, it has been known that differences in corrodibility cause individual particles to vary in life (i.e., the period of time in which the initial characteristics are maintained), and differences in ease of charging and discharging cause individual particles to vary in characteristics such as electrical characteristics.
Consequently, it has been the existing state of the art that, when an alloy powder is used in the form of a mass of some size (e.g., an electrode), the characteristics (such as life and charge-discharge capacity) of the electrode depend on the lower limits of the characteristics of the alloy powder used therein and, therefore, the capabilities inherently possessed by the alloy powder are not fully brought out.
As a characteristic feature of a hydrogen absorbing alloy, changes in volume due to the absorption and release of hydrogen produce cracks. However, when an alloy powder is produced by mechanical impact pulverization, the alloy may be cracked at places different from those where cracks are produced by changes in volume. Consequently, when the cycle of hydrogen absorption and release is repeated many times, changes in volume due to the absorption and release of hydrogen produce cracks. This causes a further particle size reduction of the powder and hence accelerates the deterioration of the above-described characteristics.
In order to cope with the above-described situation, various measures for the improvement of corrosion resistance have been taken thus far. One example thereof is to suppress the corrosion of a mechanically ground alloy powder by applying a highly anticorrosive protective film thereto (e.g., plating the alloy powder with nickel) and thus preventing the electrolyte from coming into direct contact with the alloy. Another example is to treat alloy particles with a highly corrosive fluid (e.g., a hot aqueous alkaline solution), whereby the components which will dissolve upon contact with the electrolyte are leached out in advance and the elements which are highly resistant to corrosion by the electrolyte (e.g., nickel) are selectively left so as to serve as a protective film. Still another example is to prepare a homogeneous alloy powder by heat-treating a cast alloy so as to reduce the segregation of components in the alloy and homogenize its composition, and then grinding it mechanically, so that alloy components are prevented from being locally dissolved out upon direct contact with the electrolytic solution.
However, the above-described prior art methods have been almost ineffective in eliminating the variation in characteristics resulting from differences in alloy particle size and in preventing cracking and particle size reduction induced in the alloy by the absorption and release of hydrogen.
The reason for this is that all methods involve grinding an alloy by the action of mechanical impact and hence fail to cope with both of the problems arising from cracking and particle size reduction induced by the absorption and release of hydrogen, and the problems arising from a wide particle size distribution extending from fine particles to coarse particles, i.e., the above-described problems arising from the variation in characteristics resulting from differences in particle size.